1. Field of the Invention
The invention relates to electrical switching apparatus, such as, for example, circuit breakers and, more particularly, to circuit breakers employing a slot motor. The invention also relates to methods of installing slot motor assemblies in circuit interrupters.
2. Background Information
Circuit interrupters, such as circuit breakers, are employed in diverse capacities in power distribution systems. A circuit breaker may include, for example, a line conductor, a load conductor, a fixed contact and a movable contact, with the movable contact being movable into and out of electrically conductive engagement with the fixed contact. This switches the circuit breaker between an on or closed position and an off or open position, or between the on or closed position and a tripped or tripped off position. The fixed contact is electrically conductively engaged with one of the line and load conductors, and the movable contact is electrically conductively engaged with the other of the line and load conductors. The circuit breaker may also include an operating mechanism having a movable contact arm upon which the movable contact is disposed.
In order to enhance the speed of separation of the separable contacts, the contacts may be disposed within a slot motor, which increases interruption performance. Ring-shaped or loop-shaped slot motors typically have two assemblies, an upper assembly and a lower assembly. Both of the upper and lower assemblies include a corresponding insulative housing and a plurality of plates composed of magnetically permeable material (e.g., steel), which surrounds the separable contacts and the movable contact arm of the circuit breaker. The lower assembly is disposed below the fixed contact. When the power circuit is live, an electrical arc may be drawn between the separable contacts during separation. The electrical current interacts electromagnetically with the slot motor to induce a magnetic field in the magnetic material of the slot motor, which, in turns, interacts with the separating contacts and the movable contact arm to accelerate the contact opening process. Examples of slot motors are disclosed in U.S. Pat. Nos. 4,375,021; 4,546,336; 4,546,337; 4,549,153; 4,970,482; 5,694,098, and 6,281,459.
As shown in FIG. 1, the upper assembly is an inverted U-shaped assembly having a housing assembly 1 and a plurality of plates 2, forming a bight portion 3 and two legs 4,5. The upper slot motor assembly is structured to be disposed over the movable contact (not shown) wherein the tips of the upper assembly legs 4,5 contact the lower slot motor assembly (not shown). The upper assembly legs 4,5 have an extended length to accommodate the path of travel of the movable contact arm (not shown). That is, the movable contact (not shown) is disposed between the upper assembly legs 4,5 and as the movable contact moves between the first, open position and the second, closed position, the movable contact moves from a position adjacent to the upper assembly bight portion 3 to a position adjacent the tips of the legs 4,5. Accordingly, the legs 4,5 have a sufficient length to accommodate the path of travel of the movable contact arm.
FIG. 2 shows a circuit breaker 6 including a housing 7, separable contacts 8,9 enclosed by the housing 7, and a spring powered operating mechanism 10 which opens the separable contacts 8,9 to interrupt the current through the conductors of an electrical system (not shown) in response to electrical fault conditions. The circuit breaker 6 also includes a loop-shaped slot motor 12 and an arc chute 14. The separable contacts 8,9 generally comprise one or more movable contacts 8 and one or more corresponding stationary contacts 9. Each movable contact 8 is disposed at or about a first end 16 of a spring-biased movable contact arm 18. The spring-biased movable contact arm 18 is pivotably coupled, at or about its second end 20, to a crossbar 22 of the operating mechanism 10. The crossbar 22 carries the movable contact arms 18 for all of the poles 24 (only one pole 24 is shown) of the circuit breaker 6, and cooperates with a cradle 26 of the circuit breaker operating mechanism 10 to allow for simultaneous opening and closing of the contacts 8,9 in all of the poles 24.
The operating mechanism 10 controls the spring-biased movable contact arm 18 to pivot the movable contact 8 into and out of electrical contact with the corresponding stationary contact 9. A contact arm spring 28 biases the second end 20 of the movable contact arm 18, proximate the operating mechanism crossbar 22, in order to maintain the closed position (shown in phantom line drawing) of the pair of movable and stationary contacts 8,9.
A slot motor having a relatively narrow width channel is essential for effective current-limiting and arc quenching. However, assembly of a narrow width channel slot motor becomes a manufacturing challenge since the narrow width channel and the shape of the reverse loop conductor prevent assembly. For example, in one prior proposal, such as the circuit breaker 6 of FIG. 2, a copper reverse loop conductor 30 is bent upward (not shown) to allow the slot motor 12 to slide around the conductor 30. Then, the copper conductor 30 is re-bent back to its intended position (as shown in FIG. 2). The bending stresses the copper conductor 30, which, generally, cannot be reliably re-bent back to the proper position especially with the slot motor 12 in place.
There is room for improvement in electrical switching apparatus, such as circuit breakers, employing a slot motor.
There is also room for improvement in methods of installing slot motor assemblies in circuit interrupters.